Flu Season Starts Early

By Eric Toner, MD, December 7, 2012

The most recent release of influenza surveillance data from the CDC on November 30, 2012, indicates an unexpectedly early start to the US flu season.1 The data for week 47, which ended on November 24 (the Saturday after Thanksgiving), indicated a rise in influenza-like illness (ILI) above the regional baseline in 5 of 10 HHS regions of the US. It was accompanied by a rise in the percentage of positive influenza tests in the same regions.

This is the earliest start of a flu season in the US since the memorable 2003-2004 season, which was remarkable for being early and quite severe, with an unusually high number of pediatric deaths. In that 2003-2004 season,2 the predominant strain was an H3N2 variant referred to as the Fujian strain that was not well matched to that year’s vaccine. This year, the predominant strain is also an H3N2 (Victoria/361/2011-like), but it appears to be well matched to this year’s vaccine. Generally, in flu seasons in which H3N2 predominates, illness tends to be more severe than in other years.

Epidemic Starting in the South

The regions experiencing early flu activity are primarily in the South, with the highest levels of ILI reported in Texas, Louisiana, Mississippi, Alabama, and Tennessee. As expected this early in the season, the number of flu related deaths (pediatric influenza-related deaths and pneumonia and influenza mortality) has not yet risen. Flu-related hospitalization data has not yet been reported.

Of the isolates typed so far this season, 70% are influenza A; of those subtyped, 199/200 are H3. Only one H1N1 isolate has been found. In the southern regions where rates of ILI have been highest, the percentage of influenza B isolates has been higher than the national average, but the significance of this development is not yet clear.

No NI Resistance

Antiviral resistance testing has found no evidence of neuraminidase inhibitor (NI)—oseltamivir and zanamivir—resistance so far this season; however, adamantane (amantadine and rimantadine) resistance remains widespread.

Reminder to Clinicians

Influenza vaccination is recommended for people in all age groups older than 6 months. It takes approximately 2 weeks for vaccine to produce reliable protection; therefore, clinicians should be strongly encouraging vaccination now. Also important to remember is the recommendation for early treatment with antivirals, especially for patients with severe, complicated, or progressive disease and for those at higher than normal risk for complications. The next update from the CDC will be released on December 7.